The invention relates to a method of calculating engine output and an apparatus that calculates engine output according to the calculation method. 2. Description of the Related Art
Recently, it has become necessary to control internal combustion engines to further increase output, improve fuel efficiency, and reduce exhaust emissions. For example, Japanese Patent Application Publication No. JP-A-2002-4928 describes setting ignition timing while executing a model-based control that controls an actual air-fuel ratio to be a target air-fuel ratio. In particular, the ignition timing is set using a rich air-fuel ratio map when the target air-fuel ratio is rich and a lean air-fuel ratio map when the target air-fuel ratio is lean. If the target air-fuel ratio is between the values in the rich air-fuel ratio map and the values in the lean air-fuel ratio map, the ignition timing is set by applying linear interpolation between the two maps.
However, it is difficult to accurately determine the characteristic of the parameter relating to the operating condition of the engine, such as ignition timing, by applying linear interpolation between a plurality of maps. Thus, in the method described above, the value of the parameter determined by interpolation is generally inaccurate. Therefore, when the value of the parameter is determined by interpolation, the engine is operated based on an inaccurate value of the parameter, which decreases the reliability of the control.
Torque estimation models or maps used to estimate the torque of the engine may be made under various operating conditions to improve the accuracy of engine control. In this case, however, a greatly increased number of measurements must be taken, and the number of man-hours required to obtain such measurements increases accordingly. Because of the increased number of man-hours required to make the torque estimation models or the maps during the development of an internal combustion engine, the start of the analysis of the engine control is delayed. As a result, the development period of the engine is increased.
For example, the relation between the ignition timing and the torque is indicated by a curve represented by a quadratic equation or a quartic equation when the operating condition remains constant (i.e., the air-fuel ratio, engine speed, load factor, and valve timing remain the same). If the air-fuel ratio, engine speed, or load factor changes, the shape of the curve also changes. Therefore, if the maps are made using only data obtained empirically, the relation between the ignition timing and the torque needs to be measured a plurality of times under different operating conditions while the air-fuel ratio, engine speed, and load factor are changed. This increases the number of man-hours required to perform the measurements.